Axially flexible stent

ABSTRACT

A stent with axial flexibility, in a preferred embodiment, has a longitudinal axis and comprises a plurality of longitudinally disposed bands, wherein each band defines a generally continuous wave along a line segment parallel to the longitudinal axis. A plurality of links maintains the bands in a tubular structure. In a further embodiment of the invention, each longitudinally disposed band of the stent is connected, at a plurality of periodic locations, by a short circumferential link to an adjac6ent band.

CROSS REFERENCE

This application is a continuation-in-part of U.S. application Ser. No.08/770,236 filed on Dec. 20, 1996 and U.S. application Ser. No.08/835,222, filed Apr. 7, 1997, both applications herein incorporated byreference. Ser. No. 08/770,236 bears priority from provisionalapplications application Ser. No. 60/010,686, filed Jan. 26, 1996, nowabandoned; Ser. No. 60/017,479, filed Apr. 26, 1996, now abandoned; Ser.No. 60/017,415, filed May 8, 1996; Ser. No. 60/024,110, filed Aug. 16,1996, all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a stent having axial flexibility andresilience in its expanded form.

BACKGROUND ART

A stent is commonly used as a tubular structure left inside the lumen ofa duct to relieve an obstruction. Commonly, stents are inserted into thelumen in a non expanded form and are then expanded autonomously (or withthe aid of a second device in situ. A typical method of expansion occursthrough the use of a catheter mounted angioplasty balloon which isinflated within the stenosed vessel or body passageway in order to shearand disrupt the obstructions associated with the wall components of thevessel and to obtain an enlarged lumen.

In the absence of a stent, restenosis may occur as a result of elasticrecoil of the stenotic lesion. Although a number of stent designs havebeen reported, these designs have suffered from a number of limitations.These include restrictions on the dimension of the stent such asdescribes a stent which has rigid ends (8 mm) and a flexible median partof 7-21 mm. This device is formed of multiple parts and is notcontinuously flexible along the longitudinal axis. Other stent designswith rigid segments and flexible segments have also been described.

Other stents are described as longitudinally flexible but consist of aplurality of cylindrical elements connected by flexible members. Thisdesign has at least one important disadvantage, for example, accordingto this design, protruding edges occur when the stent is flexed around acurve raising the possibility of inadvertent retention of the stent onplaque deposited on arterial walls. This may cause the stent to embolizeor more out of position and further cause damage to the interior liningof healthy vessels. (See FIG. 1(a) below).

Thus, stents known in the art, which may be expanded by balloonangioplasty, generally compromise axial flexibility to permit expansionand provide overall structural integrity.

SUMMARY OF THE INVENTION

The present invention overcomes some perceived shortcomings of prior artstents by providing a stent with axial flexibility. In a preferredembodiment, the stent has a first end and a second end with anintermediate section between the two ends. The stent further has alongitudinal axis and comprises a plurality of longitudinally disposedbands, wherein each band defines a generally continuous wave along aline segment parallel to the longitudinal axis. A plurality of linksmaintains the bands in a tubular structure. In a further embodiment ofthe invention, each longitudinally disposed band of the stent isconnected, at a plurality of periodic locations, by a shortcircumferential link to an adjacent band. The wave associated with eachof the bands has approximately the same fundamental spatial frequency inthe intermediate section, and the bands are so disposed that the wavesassociated with them are spatially aligned so as to be generally inphase with one another. The spatially aligned bands are connected, at aplurality of periodic locations, by a short circumferential link to anadjacent band.

In particular, at each one of a first group of common axial positions,there is a circumferential link between each of a first set of adjacentpairs of bands.

At each one of a second group of common axial positions, there is acircumferential link between each of a second set of adjacent rows ofbands, wherein, along the longitudinal axis, a common axial positionoccurs alternately in the first group and in the second group, and thefirst and second sets are selected so that a given band is linked to aneighboring band at only one of the first and second groups of commonaxial positions.

In a preferred embodiment of the invention, the spatial frequency of thewave associated with each of the bands is decreased in a first endregion lying proximate to the first end and in a second end region lyingproximate to the second end, in comparison to the spatial frequency ofthe wave in the intermediate section. In a further embodiment of theinvention, the spatial frequency of the bands in the first and secondend regions is decreased by 20% compared with the spatial frequency ofthe bands in the intermediate section. The first end region may belocated between the first end and a set of circumferential links lyingclosest to the first end and the second end region lies between thesecond end and a set of circumferential links lying closest to thesecond end. The widths of corresponding sections of the bands in theseend regions, measured in a circumferential direction, are greater in thefirst and second end regions than in the intermediate section. Each bandincludes a terminus at each of the first and second ends and theadjacent pairs of bands are joined at their termini to form a closedloop.

In a further embodiment of the invention, a stent is provided that hasfirst and second ends with an intermediate section therebetween, thestent further having a longitudinal axis and providing axialflexibility. This stent includes a plurality of longitudinally disposedbands, wherein each band defines a generally continuous wave having aspatial frequency along a line segment parallel to the longitudinalaxis, the spatial frequency of the wave associated with each of thebands being decreased in a first end region lying proximate to the firstend and in a second end region lying proximate to the second end, incomparison to the spatial frequency of the wave in the intermediatesection; and a plurality of links for maintaining the bands in a tubularstructure. The first and second regions have been further defined as theregion that lies between the first and second ends and a set ofcircumferential links lying closest to the first end and second end.

In a further embodiment the widths of the sectionals of the bands,measured in a circumferential direction, are greater in the first andsecond end regions than in the intermediate section.

In yet an additional embodiment, the stent is divided into a group ofsegments, and each of the segments are connected by a flexibleconnector. In addition, the stent segments are provided with enhancedflexibility at the flexible connectors, due to the geometricalconfiguration of the flexible connectors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects of the invention will be more readily understoodby reference to the following detailed description, taken with theaccompanying drawings, in which:

FIGS. 1(a) and 1(b) are side views of a stent having circumferentiallydisposed bands wherein the stent is in axially unbent and bent positionsrespectively, the latter showing protruding edges.

FIGS. 1(c) and 1(d) are side views of an axially flexible stent inaccordance with the present invention wherein the stent is in unbent andbent positions respectively, the latter displaying an absence ofprotruding edges.

FIG. 2 is a side view of a portion of the stent of FIGS. 1(c) and 1(d)showing the longitudinal bands, spaces, and inner radial measurements ofbends in the bands being measured in inches.

FIGS. 3(a) and 3(b) show a portion of the stent of FIG. 2 with two bandsbetween two circumferential links (a) before expansion in the undeformedstate; and (b) after expansion, in the deformed state.

FIG. 4 is a view along the length of a piece of cylindrical stent (endsnot shown) prior to expansion showing the exterior surface of thecylinder of the stent and the characteristic banding pattern.

FIG. 5 is an isometric view of a deflection plot where the stent of FIG.2 is expanded to a larger diameter of 5 mm.

FIG. 6 shows a two-dimensional layout of the stent of FIG. 4 to form acylinder such that edge "A" meets edge "B", and illustrating thespring-like action provided in circumferential and longitudinaldirections.

FIG. 7 shows a two dimensional layout of the stent. The ends aremodified such that the length (L_(A)) is about 20% shorter than length(L_(B)) and the width of the band A is greater than the width of band B.

FIG. 8 shows a perspective view of a stent containing flexibleconnectors as described in the present invention.

FIG. 9 shows a stent in which the flexible connectors are attached tostent segments, in layout form. These flexible connectors are attachedin an every-other-segment pattern.

FIG. 10 shows a layout view where the stent segments are connected witha flexible connector in every stent segment pattern.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Improvements afforded by embodiments of the present invention include(a) increased flexibility in two planes of the non-expanded stent whilemaintaining radial strength and a high percentage open area afterexpansion; (b) even pressure on the expanding stent that ensures theconsistent and continuous contact of expanded stent against artery wall;(c) avoidance of protruding parts during bending; (d) removal ofexisting restrictions on maximum of stent; and reduction of anyshortening effect during expansion of the stent.

In a preferred embodiment of the invention, an expandable cylindricalstent 10 is provided having a fenestrated structure for placement in ablood vessel, duct or lumen to hold the vessel, duct or lumen open, moreparticularly for protecting a segment of artery from restenosis afterangioplasty. The stent 10 may be expanded circumferentially andmaintained in an expanded configuration, that is circumferentiallyrigid. The stent 10 is axially flexible and when flexed at a band, thestent 10 avoids any externally protruding component parts.

FIG. 1 shows what happens to a stent 10, of a similar design to apreferred embodiment herein but utilizing instead a series ofcircumferentially disposed bands, when caused to bend in a manner thatis likely encountered within a lumen of the body. A stent 10 with aeffect analogous to a series of railroad cars on a track. As the row ofrailroad cars proceeds around the bend, the corner of each carproceeding around the bend after the coupling is caused to protrude fromthe contour of the track. Similarly, the serpentine circumferentialbands have protrusions (2) above the surface of the stent 10 as thestent 10 bends.

In contrast, the novel design of the embodiment shown in FIGS. 1(c) and1(d) and FIG. 7 in which the bands (3) are axially flexible and arearranged along the longitudinal axis, avoids such an effect when thestent 10 is bent, so the bent bands (4) do not protrude from the profileof the curve of the stent 10. Furthermore, any flaring at the ends ofthe stent 10 that might occur with a stent 10 having a uniform structureis substantially eliminated by introducing a modification at the ends ofthe stent 10. This modification comprises decreasing the spatialfrequency and increasing the width of the corresponding bands in acircumferential direction (L_(A) and A) compared to that of theintermediate section. (l_(B) and B).

In an embodiment of the invention, the spatial frequency L_(A) may bedecreased 0-50% with respect to L_(B), and the width A may be increasedin the range of 0-150% with respect to B. Other modifications at theends of the stent 10 may include increasing the thickness of the wall ofthe stent 10 and selective electropolishing. These modifications protectthe artery and any plaque from modifications protect the artery and anyplaque from abrasion that may be caused by the stent 10 ends duringinsertion of the stent 10. The modification also may provide increasedradio-opacity at the ends of the stent 10. Hence it may be possible tomore accurately locate the stent 10 once it is in place in the body.

The embodiment as shown in FIGS. 2 and 6 has the unique advantage ofpossessing effective "springs" in both circumferential and longitudinaldirections shown as items (5) and (6) respectively. These springsprovide the stent 10 with the flexibility necessary both to navigatevessels in the body with reduced friction and to expand at the selectedsite in a manner that provides the final necessary expanded dimensionswithout undue force while retaining structural resilience of theexpanded structure.

As shown in both FIGS. 2, 4 and 6, each longitudinal band undulatesthrough approximately two cycles before there is formed acircumferential link to an adjacent band. Prior to expansion, the wave Wassociated with each of the bands may have approximately the samefundamental spatial frequency, and the bands are so disposed that thewave W associated with them are spatially aligned, so as to be generallyin phase with one another as shown in FIG. 6.

The aligned bands on the longitudinal axis are connected at a pluralityof periodic locations, by a short circumferential link to an adjacentband. Consider a first common axial position such as shown by the lineX--X in FIGS. 4 and 6. Here an adjacent pair of bands is joined bycircumferential link 7. Similarly other pairs of bands are also linkedat this common axial position. At a second common axial position, shownin FIG. 6 by the line Y--Y, an adjacent pair of bands is joined bycircumferential link 8. However, any given pair of bands that is linkedat X--X is not linked at Y--Y and vice-versa. The X--X pattern oflinkages repeats at the common axial position Z--Z. In general, thereare thus two groups of common axial positions. In each of the axialpositions of any one group are links between the same pairs of adjacentbands, and the groups alternate along the longitudinal axis of theembodiment. In this way, circumferential spring 6 and the longitudinalspring 6 are provided.

A feature of the expansion event is that the pattern of open space inthe stent 10 of the embodiment of FIG. 2 before expansion is differentfrom the pattern of the stent 10 after expansion. In particular, in apreferred embodiment, the pattern of open space on the stent 10 beforeexpansion is serpentine, whereas after expansion, the pattern approachesa diamond shape (3a, 3b). In embodiments of the invention, expansion maybe achieved using pressure from an expanding balloon or by othermechanical means.

In the course of expansion, as shown in FIG. 3, the wave W shaped bandstend to become straighter. When the bands become straighter, they becomestiffer and thereby withstand relatively high radial forces. FIG. 3shows how radial expansion of the stent 10 causes the fenestra to openup into a diamond shape with maximum stress being expended on the apicesof the diamond along the longitudinal axis. When finite element analysesincluding strain studies were performed on the stent 10, it was foundthat maximum strain was experienced on the bands and links and was belowthe maximum identified as necessary to maintain structural integrity.

The optimization of strain of the stent 10 is achieved by creating aslarge a turn radius as possible in the wave W associated with each bandin the non-expanded stent 10. This is accomplished while preserving asufficient number of bands and links to preserve the structuralintegrity of the stent 10 after expansion. In an embodiment of theinvention, the strain may be less than 0.57 inches/inch for 316Lstainless steel. The expansion pressure may be 1.0-7.0 atmospheres. Thenumber of bands and the spatial frequency of the wave W on thelongitudinal axis also affects the number of circumferential links. Thecircumferential links contribute structural integrity during applicationof radial force used in expansion of the stent 10 and in the maintenanceof the expanded form. While not being limited to a single set ofparameters, an example of a stent 10 of the invention having alongitudinal axis and providing axial flexibility of the type shown inFIG. 6, may include a stent 10 having an expanded diameter of 4 mm and alength of 30 mm that for example may have about 8-12 rows, moreparticularly 10 rows and about 6-10 slots, more particularly 8 slots (aslot is shown in FIG. 6 as extending between X and Z), with a wave Wamplitude of about 1/4-1/10 of a slot length, more particularly 1/8 of aslot length.

The stent 10 may be fabricated from many methods. For example, the stent10 may be fabricated from a hollow or formed stainless steel tube thatmay be cut out using lasers, electric discharge milling (EDM), chemicaletching or other means. The stent 10 is inserted into the body andplaced at the desired site in an unexpanded form. In a preferredembodiment, expansion of the stent 10 is effected in a blood vessel bymeans of a balloon catheter, where the final diameter of the stent 10 isa function of the diameter of the balloon catheter used.

In contrast to stents of the prior art, the stent 10 of the inventioncan be made at any desired length, most preferably at a nominal 30 mmlength that can be extended or diminished by increments, for example 1.9mm increments.

It will be appreciated that a stent 10 in accordance with the presentinvention may be embodied in a shape memory material, including, forexample, an appropriate alloy of nickel and titanium; or stainlesssteel. In this embodiment after the stent 10 has been formed, it may becompressed so as to occupy a space sufficiently small as to permit itsinsertion in a blood vessel or other tissue by insertion means, whereinthe insertion means include a suitable catheter, or flexible rod. Onemerging from the catheter, the stent 10 may be configured to expandinto the desired configuration where the expansion is automatic ortriggered by a change in pressure, temperature or electricalstimulation.

An embodiment of the improved stent 10 has utility not only within bloodvessels as described above but also in any tubular system of the bodysuch as the bile ducts, the urinary system, the digestive tube, and thetubes of the reproductive system in both men and women.

In yet a further embodiment, there is described a stent 10 as presentlydisclosed containing a multiplicity of curvilinear segments 20. Thesecurvilinear segments 20 are connected to each other via a generallyperpendicular connector 25. The generally perpendicular connector 25lies substantially in the plane perpendicular to the longitudinal axisof the stent 10. Each of the stent 10 segments as described herein isconnected to an adjacent stent 10 segment. This is done using a seriesof flexible connectors. Importantly, the connectors themselves can bemade narrower at their midpoints. This enhances the possibility offlexure at that point. Of course, it is to be realized that alternatedesigns of the connector to insure flexibility are possible, andcontemplated by this invention.

In essence therefore, the stent 10 as described in FIG. 8 is a stent 10of considerable flexibility when compared to more rigid rectilinearstents. Nonetheless, the stent 10 of the present invention does notdepart from the basic concepts set forth herein, in that it discloses acontinuously curvilinear strut. This curvilinear strut is connected toother curvilinear struts via a series of "second" more flexibleconnectors, described above.

In any regard, it can be seen that the stent 10 of the present inventionincorporates various new and useful members. One of them is the flexibleconnector in conjunction with a generally curvilinear stent. Another isthe use of the generally larger struts at the ends of the stent 10 inorder to provide for continued support at the stent 10 ends. A finalaspect is the use of flexible connectors amongst stent 10 segments toprovide for greater flexibility.

In all regards, however, it is to be seen that the present invention isto be determined from the attached claims and their equivalents.

What is claimed is:
 1. A stent having first and second ends with anintermediate section therebetween, the stent further having alongitudinal axis and providing axial flexibility, comprising:aplurality of longitudinally disposed bands, wherein each band defines agenerally continuous wave having a spatial frequency along a linesegment parallel to the longitudinal axis; a plurality of links formaintaining the bands in a tubular structure, wherein the links are sodisposed that any single circumferential path formed by the links isdiscontinuous; and wherein the stent comprises a plurality of stentsegments, each of the stent segments containing at least one generallycontinuous curvilinear strut, said stent segments connected by at leastone flexible connector displaced between a pair of adjacent stentsegments.
 2. A stent according to claim 1, wherein each link is axiallydisplaced from any circumferentially adjacent link.
 3. A stent accordingto claim 1, wherein the wave associated with each of the bands hasapproximately the same fundamental spatial frequency for theintermediate section.
 4. A stent according to claim 3, wherein the bandsare so disposed that the waves associated with them are spatiallyaligned so as to be generally in phase with one another.
 5. A stentaccording to claim 4, wherein each link is axially displaced from anycircumferentially adjacent link.
 6. A stent according to claim 5,wherein, at each one of a first group of common axial positions, thereis a circumferential link between each of a first set of adjacent pairsof bands.
 7. A stent according to claim 2, wherein the spatial frequencyof the wave associated with each of the bands, is decreased in a firstend region lying proximate to the first end and in a second end regionlying proximate to the second end, in comparison to the spatialfrequency of the wave in the intermediate section.
 8. A stent accordingto claim 7, wherein the spatial frequency is decreased by about 20%compared with the spatial frequency of the wave in the intermediatesection.
 9. A stent according to claim 7, wherein the first end regionlies between the first end and a set of circumferential links lyingclosest to the first end and the second end region lies between thesecond end and a set of circumferential links lying closest to thesecond end.
 10. A stent according to claim 7, wherein widths ofcorresponding sections of the bands, measured in a circumferentialdirection, are greater in the first and second regions than in theintermediate section.
 11. A stent according to claim 9, wherein widthsof corresponding sections of the bands, measured in a circumferentialdirection, are greater in the first and second regions than in theintermediate section.
 12. A stent according to claim 8, wherein thefirst end region lies between the first end and a set of circumferentiallinks lying closest to the first end and the second end region liesbetween the second end and a set of circumferential links lying closestto the second end.
 13. A stent according to claim 12, wherein widths ofcorresponding sections of the bands, measured in a circumferentialdirection, are greater in the first and second end.
 14. A stentaccording to claim 6, wherein the spatial frequency of the waveassociated with each of the bands, is decreased in a first end regionlying proximate to the first end and a second end region lying proximateto the second end, in comparison to the spatial frequency of the wave inthe intermediate section.
 15. The stent of claim 1 wherein the flexibleconnector is generally an "Ω" shape.